The invention relates to a mass flowmeter comprising a hollow conduit of a heat-conducting material for transporting in a determined flow direction a fluid with a mass flow rate to be measured, a first temperature-sensitive resistor element at a first position in thermal contact with this conduit for supplying heat to said fluid, a temperature sensor and measuring and control means connectable to the resistor element and the temperature sensor.
Such a mass flowmeter is known from the U.S. Pat. No. 4,984,460, wherein a first temperature-sensitive resistor element wound round a conduit tube is incorporated in a first bridge circuit, which further comprises two resistor elements which function respectively as temperature sensor for determining the ambient temperature and as setting resistor for setting a temperature of the conduit tube at the position of the first resistor element through heat dissipation in this resistor element. The known mass flowmeter further comprises a second temperature-sensitive resistor element which is wound round the conduit tube and which is incorporated in a second bridge circuit, which likewise further comprises two resistor elements which function respectively as temperature sensor for determining the ambient temperature and as setting resistor for setting a temperature of the conduit tube at the position of the second resistor element through heat dissipation in this resistor element. The bridge circuits are connected to a control unit which arranges that the difference in temperature between the two resistor elements wound round the conduit tube and the ambient temperature is roughly equal to a value set using the setting resistors. The mass flow rate of a fluid flowing through the conduit tube is determined in the known device from the difference in energy supplied to the first and second resistor elements wound round the conduit tube.
Because use has to be made of two bridge circuits for operation of the mass flowmeter known from the U.S. patent, the difference in temperature at the position of the first and second resistor elements wound round the conduit tube does not always equal the value zero, which results in an inherent limitation to the sensitivity of this mass flowmeter. A further drawback of the use of two bridge circuits, in addition to inherent stability problems, ensues from the relatively large number of components required for these circuits, this having a cost-increasing effect.
It is an object of the invention to provide a mass flowmeter with a simple principle of operation, wherein it is possible in principle to suffice with one bridge circuit.
It is also an object to provide a mass flowmeter with a greater range than the known mass flowmeter.
It is a further object of the invention to provide a mass flowmeter with which the mass flow rate of a fluid can be measured more quickly and accurately than with a prior art mass flowmeter.
The said objectives are also stated in the European Patent Application EP 0467430A. A known, conventional flowmeter is also described hereinxe2x80x94see column 1, lines 33-48. Stated as drawbacks of this known flowmeter arexe2x80x94see column 1, lines 49-56-: xe2x80x98Non-linear relationship between the flow and the sensor-output, showing a small inclination and an inflection point in the range of small flow, wherein the flow rate approaches zero, thereby indicating the decreased sensitivity of the sensor in that rangexe2x80x99. In column 2, line 31 up to and including column 4, line 5 and referring to the drawings 1-3 accompanying that patent application, there is explained in detail what a conventional thermal-type flowmeter looks like, and in column 4, line 31 and further, referring to FIGS. 4, 5 and 6, there is explained how a thermal-type flowmeter can be constructed to overcome the said drawbacks and be highly sensitive, even in the range of small flows, and be capable of generating a high sensor output over the full range of flow rates, with no inflection point.
These objectives are achieved, and other advantages gained, with a mass flowmeter of the type stated in the preamble, wherein according to the invention the temperature sensor is provided in thermal contact with the conduit at a second position upstream in relation to said first position, and the measuring and control means are adapted to maintain a constant difference in temperature at said first and second positions.
The invention is based on the surprising insight that the mass flow rate of a fluid flowing through a hollow conduit can be derived in reliable manner from the energy which must be supplied at said second position to maintain the difference in temperature at said first and second positions at a constant value.
In an embodiment of a mass flowmeter according to the invention the temperature sensor is a second heat-sensitive resistor element.
In a favourable embodiment the first and the second heat-sensitive resistor element have the same temperature coefficient and these elements are incorporated in a bridge circuit, wherein the resistance of the second resistor element is greater at a determined temperature than the resistance of the first resistor element. Otherwise than in the prior art device, the output signal of the bridge circuit in this embodiment, which is a measure for the mass flow rate to be measured, is wholly independent of the temperature of a medium flowing through the conduit.
In a preferred embodiment the bridge circuit is a wheatstone bridge, the output of which is connected with a feedback loop to the top of the bridge.
In a subsequent embodiment a mass flowmeter with wheatstone bridge comprises a third temperature-sensitive resistor element identical to the first temperature-sensitive resistor element at a third position downstream in relation to said second position in thermal contact with this conduit for supplying heat to the fluid. By choosing identical first and third resistor elements and heating the temperature thereof to the same constant value above the value measured by the temperature sensor, the output signal of the wheatstone bridge is, other than in the preceding embodiments, equal to zero if the mass flow rate for measuring is zero, so that it is not necessary to correct for an offset signal. If the conduit is moreover configured in this embodiment such that heat dissipated in the first and third resistor elements can disappear via convection, conduction or radiation without influencing the value measured by the temperature sensor, the sign of the output signal of the wheatstone bridge moreover gives information about the flow direction of the fluid through the conduit.
In a following embodiment a mass flowmeter with wheatstone bridge comprises an additional temperature-sensitive resistor element identical to the second temperature-sensitive resistor element at a fourth position downstream in relation to said second and first position in thermal contact with this conduit for measuring the temperature of the fluid. This embodiment provides the advantage that the value of the temperature measured by the first temperature sensor can be replaced by an average of the values measured by the first and second temperature sensor, so that inaccuracies in the measured value of the temperature resulting from temperature gradients on the conduit are to a large extent averaged out. If the conduit is moreover configured in this embodiment such that heat dissipated in the first resistor element can disappear via convection, conduction or radiation without influencing the values measured by the temperature sensors, the sign of the output signal of the wheatstone bridge moreover gives information about the flow direction of the fluid through the conduit.
In a very favourable embodiment a mass flowmeter with wheatstone bridge comprises a third temperature-sensitive resistor element identical to the first temperature-sensitive resistor element at a third position downstream in relation to said second position in thermal contact with this conduit for supplying heat to the fluid, and a fourth temperature-sensitive resistor element identical to the second temperature-sensitive resistor element at a fourth position downstream in relation to said second, first and third position in thermal contact with this conduit for measuring the temperature of the fluid. The advantages of the two latter described embodiments are combined in this embodiment.
It will be apparent that the concepts of xe2x80x9cupstreamxe2x80x9d and xe2x80x9cdownstreamxe2x80x9d in respect of the described latter three embodiments have an arbitrary significance and serve only to designate the first, second, third and fourth positions relative to each other.
In order to eliminate or at least reduce to a significant extent errors in a mass flow rate to be measured resulting from self-heating of the second resistor element functioning as temperature sensor, the resistance of the second resistor element is greater at a determined temperature than the resistance of the first resistor element, preferably by at least a factor of 10.
In a favourable embodiment the first and the second heat-sensitive resistor element are platinum resistors.
The heat-conducting material of a hollow conduit in a mass flowmeter according to the invention preferably has a thermal conduction coefficient xcex with a value at least equal to 10 W.mxe2x88x921.Kxe2x88x921. The adverse effect on the result of the flow rate measurement as a consequence of a possible self-heating of the temperature sensor is further suppressed with such a material, for instance stainless steel.
In a practical embodiment the hollow conduit comprises a tube, the inner diameter of which lies in the range between about 0.1 mm and 5 mm, preferably in the range between about 0.8 mm and 3 mm.
The conduit has a wall thickness for instance in the range between about 0.05 mm and about 0.5 mm, preferably in the range between about 0.1 mm and about 0.3 mm.
In an embodiment which is particularly suitable for a strong reduction of errors in a mass flow rate to be measured as a consequence of self-heating of the second resistor element functioning as temperature sensor, the conduit has at the location of the second, the fourth respectively the second and the fourth position a greater wall thickness than at the location of the first position. Owing to the greater wall thickness at said positions, i.e. at the position of the first and/or second temperature sensor, a larger contact surface between conduit and temperature sensor is available for discharging the minimal quantity of heat dissipated in the sensor, while the mass of the underlying thicker conduit wall moreover functions as heat discharge.
In an alternative embodiment of a mass flowmeter according to the invention the temperature sensor is a thermo-element, preferably a thermopile, a first side of which is thermally coupled to said first position and a second side of which is thermally coupled to said second position.
When a thermo-element is applied as temperature sensor, the measuring and control means comprise, by way of example, a per se known processor.
It is noted that the temperature sensor is not limited to the above stated embodiments; this sensor can in principle comprise any temperature-sensitive element suitable for the purpose, such as a wire resistor, a thin-film resistor, a vapour-deposited or a sputtered layer, a thermistor or a pn-semiconductor transition.
A mass flowmeter as described above and also as described in the above-mentioned European patent application EP 467430 A has in turn a number of drawbacks.
Firstly: in order to make the output signal insensitive to changes in the medium temperature it is important that the resistors located both upstream and downstream have exactly the same resistance temperature coefficient. This can only be ensured if both resistors are wound from wire xe2x80x9cfrom the same reelxe2x80x9d and therefore of the same wire diameter. If in this manner the resistor located upstream has to be ten times larger than the resistor located downstream, the resistance wire located upstream becomes either ten times longer or ten times thicker than the resistance wire located downstream. Both extremes are not acceptable because with the first method the sensor is not compact and with the second method the sensor does not measure the medium temperature properly.
The drawback of this configuration is thus that the output signal of the flowmeter is dependent on the medium temperature.
Secondly: it is the intention that the resistor located upstream detects only the medium temperature. By placing the resistor located downstreamxe2x80x94the heaterxe2x80x94far enough away from the resistor located upstreamxe2x80x94the sensorxe2x80x94, it is possible to prevent heat leaking from the heater reaching the sensor. However, by incorporating both resistors in a wheatstone bridge configuration, the current through the sensor will, because the current through the heater increases as the flow increases, also increase. This latter causes the sensor to be also heated by dissipation of energy and to also begin to function as heater. As the flow increases the sensitivity of the flow sensor will hereby decrease relative to the theoretically expected sensitivity, and the measuring range of the sensor is limited.
The drawback of this configuration is therefore that the sensitivity of the flow sensor depends on the flow, whereby the measuring range is bounded at the upper limits.
Thirdly: a problem related to the foregoing is that the flow sensor responds more slowly to changes in the flow. This is caused by both sensor and heater functioning as heater: both generate heat to the flow. This involves two time constants, so that the flow sensor will only indicate the final value of the flow after a longer period than if only the heater were generating heat to the flow.
The drawback of this configuration is therefore that the response speed of the flow sensor is not optimal. There may even occur a xe2x80x9clag effectxe2x80x9d (first a rapid response resulting from the heater, then a slow response to the final value as a result of the sensor).
Further objects of the invention are therefore:
to provide a mass flowmeter with which higher flows can be measured than with existing flowmeters;
to provide a mass flowmeter with an output signal which is independent of the medium temperature;
to enable more accurate measurement in that the meter operates for longer in accordance with theory;
to enable more rapid measurement in that only the heater has to generate heat and the sensor no longer does so, so that a xe2x80x9clag effectxe2x80x9d no longer occurs.
The observed drawbacks of a flowmeter as described above can be avoided by taking the following measures in respect of the construction of the flowmeter.
The hollow conduitxe2x80x94the tubexe2x80x94can be thickened at the position of the upstream side resistorxe2x80x94the sensorxe2x80x94, whereby the outer surface of the tube is enlarged. A sensor resistor with a higher value than the downstream side resistorxe2x80x94the heaterxe2x80x94can thus be realized in simple manner without it being much longer or thicker than that of the downstream side resistor. It is possible in this manner to satisfy the requirement that R(sensor):R(heater)=10:1 with resistance wire xe2x80x9cfrom the same reelxe2x80x9d. By combining the flow sensor realized in this manner with the circuit of FIG. 2 a mass flowmeter is constructed, the output voltage of which is independent of the medium temperature (intrinsic temperature compensation).
The temperature-sensitive resistor situated upstream, the sensor, can be replaced in the wheatstone bridge by a fixed-value resistor, whereby the output voltage of the wheatstone bridge depends on the temperatures. When the sensor is removed from the bridge, no self-heating takes place and the sensitivity of the flow sensor to flow remains constant, irrespective of the flow. The accuracy and response speed hereby also increase. By arranging the medium temperature sensor in the circuit after the point at which the output signal of the wheatstone bridge is measured it is still possible to compensate for different temperatures of the medium (extrinsic temperature compensation).
A variant hereof is that wherein the heating element is divided into twoxe2x80x94one part which continues to function as heating element and a second part which, as temperature sensor, detects the temperature of the heating element. The two temperature sensors are then incorporated in a (passive) wheatstone bridge which serves only as measuring bridge and in which no self-heating of the sensors takes place. The heating element is heated separately of the wheatstone bridge, a temperature sensor detects the temperature and the wheatstone bridge generates a signal such that the heating element is heated under all conditions to a constant temperature above that of the ambient. The voltage over the heating element remains the output signal, which is intrinsically temperature-compensated, is accurate and gives a rapid response.
Optionally arranged at the first temperature sensor is a further second heating element with which the direction of the flow can be determined.
The measuring range of flowmeters which operate in per se known manner with temperature sensors can be increased, when the hollow conduit has a thickened wall along a part of its length, by arranging the temperature-sensitive sensors on this part of the conduit wall. When both resistors each operate as both heating element and sensor, the heat developed by the heating element will leak away through the tube wall. When flow is running through the tube the upstream side tube wall is cooled and the downstream side tube wall is heated. The maximum measuring range is achieved when the upstream side tube wall is wholly cooled and the downstream side tube wall is wholly heated. A determined tube wall thickness represents a determined sensitivity and a determined range.
Thickening the tube wall decreases the sensitivity (the maximum difference in temperature falls, since more heat leakage is allowed), but the measuring range increases (only at a higher flow does the minimum temperature difference to be detected occur).
Measuring range and sensitivity can be further increased by increasing the length of the heating element as seen in the length direction of the tube.
When the resistance values of sensor and heating element are the same, it is possible to ensure by electronic means via the wheatstone bridge that the sensor resistance still has the desiredxe2x80x94for instance about ten timesxe2x80x94higher value than that of the heater resistance. The advantage hereof is that both resistors can be manufactured from the same resistance wire.
When the set requirements for a flowmeter as understood in this patent application are:
that the occurrence of a temperature gradient between sensor and heater must be prevented (so as to avoid the effect of such a temperature gradient on the output signal);
that the sensitivity of the meter must be maximized,
a preferred embodiment of a flowmeter according to the invention is characterized by:
a construction of the flowmeter (conduit tube, windings, housing) which is as symmetrical as possible;
a heater element which is as long as possible as seen in the length direction of the conduit tube;
a tube wall which is as thin as possible (maximum sensitivity, minimal heat leakage and optimal detection of the medium temperature);
a distance between the heater and the sensor which is greater than a defined minimum value (so as to prevent heating of the sensor by the heater).
In respect of the symmetry requirement, a tube bent into a U-shape is preferably used with symmetrically arranged windings thereon, while the fitting must also be symmetrical.
In respect of the thickness of the tube wall, a ratio of outer diameter and inner diameter of the tube is preferably chosen of             D      outer              D      inner        ≤  1.25
In respect of the distance between heater and sensor, this is preferably xe2x89xa74 mm en xe2x89xa610 mm.
The invention will now be elucidated hereinbelow on the basis of further embodiments and with reference to the drawings. Corresponding components are designated in the drawings with the same reference numerals.